1. Field of the Invention
The present invention relates to a support for recording sheet. More specifically, the present invention relates to a support for recording sheet which is provided with a substrate having disposed on both sides thereof a coating layer of a water-resistant resin, which support for recording sheet has excellent image sharpness (resolution) and excellent image light-fading resistance.
2. Description of the Related Art
Conventionally, support for recording sheets coated with a resin on both sides, such as photographic printing paper, thermographic paper, full color thermographic paper, and the like have been known. In particular, titanium dioxide, pigments, blueing agents (including blue pigments), fluorescent brighteners, and the like have been included in the coating layer of the paper used for photographic printing paper (U.S. Pat. No. 3,501,298).
The titanium dioxide used in this case has an effect of heightening the efficiency of light reflectance, and it is known that the more the content of the titanium dioxide is increased, the more image resolution is improved.
At the time a coating layer of a water-resistant resin is formed, the water-resistant resin which contains titanium dioxide is melted and extruded in the form of film from a slit die for a short period of time. However, when the titanium dioxide is incorporated in the resin coating in an amount of 20 wt % or more and the molten extrusion is conducted at a discharge temperature of 290.degree. C. to 350.degree. C., as has conventionally been the case, cracks (hereinafter referred to as "film cracking") are caused in the coating layer of the water-resistant resin, and stripes (hereinafter referred to as "die-lip stripes") are easily generated at the die-lip portion of the extruder.
When such film cracking occurs, the external appearance of the product not only deteriorates, but also, the product is deprived of its commercial value due to the loss of its water-resistant capability. Further, when die-lip stripes are generated, consecutive stripes are formed in the longitudinal direction on the surface of the manufactured film or laminate. Therefore, not only does the external appearance of the product deteriorate remarkably, but even during secondary processing such as drawing, the transparency of the film is non-uniform, and thus the commercial value of the product is lowered considerably.
In order to overcome such drawbacks, as the applicants of Japanese Patent Application Laid-Open (JP-A) No. 10-307367 have already noted and disclosed, by using a titanium dioxide pigment in which the particle surface of the titanium dioxide to be used has been subjected to a wet-process silane coupling agent coating treatment, the titanium dioxide pigment can easily be incorporated in an amount of 20 wt % or more in a polyolefine resin layer. Therefore, even when the water-resistant resin is extrusion molded at a melting temperature of around 325.degree. C., there are no occurrences of film cracking, die-lip stripes and the like.
The present inventors examined in detail the aforementioned high-resolution support for recording sheet. It was found that with the silane coupling agent coating treatment, although the titanium dioxide can be melted and extruded at a high temperature, the generation of die-lip stripes cannot be controlled completely since the activity of the titanium dioxide is not inhibited. It was also noted that there is a tendency for the light fading resistance of the print surface of the support for recording sheet to deteriorate.
Further, JP-A No. 10-307367 discloses that by surface treating the titanium dioxide with Al.sub.2 O.sub.3, aggregation of the titanium dioxide pigment at the time of lamination is prevented. However, when the Al.sub.2 O.sub.3 treatment is used, effects with regard to high filling capability and fading resistance remain insufficient.